U.S. Pat. No. 3,980,551 treats a waste lube oil with hydrogen in an ebullating bed of catalyst, then vacuum distills the product to produce a clean stock. An ebullating bed is used because prior art fixed bed treatment required vacuum distillation to remove metal containing sludge to prevent catalyst degradation in fixed bed operations.
U.S. Pat. No. 4,810,365 teaches hydrotreating of a used insulating oil when the used oil is contaminated with halocarbon compounds.
U.S. Pat. No. 3,919,076 teaches a process where a light, saturated hydrocarbon solvent is used to remove impurities such as high molecular weight additives, additive fragments, and oil oxidation products plus high density particulates from a used automotive lube oil or similar waste hydrocarbon oil. Subsequent hydrogenation is conducted under severe conditions of temperature and pressure.
U.S. Pat. No. 4,033,859 teaches a process wherein used oil, such as automotive crankcase oil, is pretreated by heating to above about 400.degree.-800.degree. F. while the system is maintained in the liquid phase at a pressure between 500-3500 psig for 15-60 minutes. A sludge containing insoluble degradation products, metallic compounds and water is separated by e.g., centrifugation, leaving a substantially ashless oil product which can be used as such or as a fuel oil or which can be further refined to produce a high quality lube oil.